The increased desirability of energy from solar energy collectors and concentrators and from the burning of inexpensive waste products, has increased the attractiveness of the free piston Stirling engine as a machine for directly converting heat energy to mechanical energy. Additionally, Stirling cycle heat pumps are effective machines for pumping heat from a cold mass to a warm mass. Such heat pumps are useful in refrigeration systems and heating systems. A free piston Stirling cycle engine linked to drive a Stirling cycle heat pump is an efficient, durable apparatus having a long life with low maintenance.
Although such a system can be designed to operate efficiently under a selected set of conditions including a selected load, problems are encountered when such an apparatus has a relatively fixed heat engine power but the load or power demand of the heat pump varies. For example, such a Stirling cycle apparatus, if used for heating the interior of a building, may have a relatively constant energy input but loading variation over a broad range. Such load variation may result from changes in the temperature of the exterior air or other mass from which the heat is pumped.
For example, if such an apparatus operates efficiently on a cold winter day and then the weather changes and becomes warmer, the heat pump is not required to do as much work. Less work is needed to heat the gas in the hot space of the heat pump to the desired temperature.
Because a Stirling engine with a constant input temperature has a tendency to continue delivering power at the same rate per unit of stroke it will begin to overstroke as the power demand of the heat pump becomes less. This can result in damage to the Stirling engine. For example, its overstroking may result in the power piston striking the interior walls at the end of the piston cylinder.
It is therefore an object of the present invention to provide an improvement which automatically prevents this overstroking by more closely matching the power output from the Stirling engine to the power loading demands of the Stirling heat pump as the heat pump temperatures vary.